1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) read head and more particularly to the improvement of the longitudinal magnetic bias of such a read head by a novel longitudinal biasing structure that interposes a soft magnetic layer between the GMR sensor element and a permanent magnet biasing layer.
2. Description of the Related Art
A giant magnetoresistive (GMR) read head requires longitudinal biasing of its free layer in order to operate in a linear readback region in as noise-free a manner as possible. The longitudinal biasing serves two complementary purposes: 1) it maintains the magnetic moment of the free layer at an optimal position (the bias point) during quiescence, so that rotations of the magnetic moment about that point during readback produce a linear response; 2) it stabilizes the domain structure of the free layer so that noise-producing domain variations (Barkhausen noise) are minimized. The longitudinal bias is generally supplied by permanent (“hard”) magnetic layers, incorporated within conducting lead layers, disposed to either side of the GMR sensor element and contacting the sensor element along some shape of junction boundary. Because the permanent magnet bias layer needs to be grown on a seed layer to achieve its magnetic hardness, the seed layer is necessarily interposed between the bias layer and the GMR sensor element at the junction. This separation between the bias layer and the GMR element tends to make the edges of the GMR element less magnetically stable than its center and, therefore, the edges become a source of domain noise. One approach to minimizing this problem is the lead-overlay (LOL) design, in which the edges of the biasing layer are shunted by a layer of good conductor. However, this configuration does not solve the problem, since the conductance of the shunting layer is finite compared to the conductance of the GMR sensor structure. In addition, the LOL design does not suppress the spurious readback signal, called “side-reading,” generated by the magnetic responses of the sensor edges. As the width of the sensor's active region (the “read width”) decreases with the increased demands imposed by high recording density, the side-reading becomes more detrimental to the quality of the signal. Without the LOL design, however, the edges constitute a major portion of the read width of the sensor. Thus, instabilities at the edges effectively produces instabilities of the entire sensor. It has also been discovered that the instabilities increase with the increase in separation between the biasing layer and the sensor edge. This separation cannot be arbitrarily reduced, however, because (as has been noted above) of the necessity of a seed layer for the biasing layer. Making the problem even more difficult to solve is the fact that there is an appreciable scatter in the directions of magnetization among the domains in the permanent magnet forming the biasing layer. As the read width of the sensor decreases below approximately 0.15 microns, this variation in permanent magnet domain structure produces unpredictable poor biasing and, therefore, the conventional permanent magnet material may itself be unsatisfactory for use in forming biasing layers. It appears, therefore, that a novel approach is needed to provide effective longitudinal biasing of a GMR sensor having a very narrow read width.
There is ample prior art that teaches the formation of stable longitudinal bias. Krounbi et al. (U.S. Pat. No. 4,785,366) teaches the covering of an entire magnetoresistive (MR) sensor by a layer of soft magnetic material over which there is formed an antiferromagnetic layer which couples to the soft layer by exchange coupling. The coupling provides logitudinal biasing at the passive lateral edges of the sensor and transverse biasing in the active readwidth region.
Schultz et al. (U.S. Pat. No. 5,712,565) teaches the formation of an MR sensor elements having permanent magnetic longitudinal biasing layers over which are formed current leads with slanted contacts. The provided configuration advantageously direct current in a manner that avoids the passage of current through the permanent magnetic material.
Shouji et al. (U.S. Pat. No. 5,926,348) teaches the formation of an MR head having a longitudinal biasing layer with a central flat portion and slanted portions laterally disposed about the flat portion. The MR element is positioned within the cavity formed by the biasing layer, whereby the slanted portions of the biasing layer provide the necessary domain stabilization to the MR element.
Lin (U.S. Pat. No. 6,030,753) teaches the formation of a longitudinally biased MR or GMR sensor by forming the longitudinal biasing layer as a continuous layer overlaying the entire sensor element, then forming a continuous conducting lead layer over the biasing layer and, finally, etching through a central portion to define an active readwidth.
Carey et al. (U.S. Pat. No. 6,266,218) teaches a longitudinal biasing layer formed as an antiferromagnetically coupled pair of ferromagnetic layers. The resulting layer is an effective biasing layer yet has a low product of thickness and magnetization.
Hasegawa et al. (U.S. Pat. No. 6,295,186) teach the formation of a spin valve structure in which an antiferromagnetic layer is used to provide a longitudinal bias to a ferromagnetic free layer and another antiferromagnetic layer forms a coercive force increasing layer to pin a ferromagnetic pinned layer with its magnetization in a transverse direction.
The inventions briefly described above do not address the problem associated with the inadequacy of hard magnetic layers to adequately bias a GMR sensor of extremely narrow trackwidth. Indeed many the cited inventions are quite similar to the LOL configuration which suffers from shortcomings also discussed above. None of the inventions teach a method of using hard magnetic layers for longitudinal biasing while still retaining the ability to fabricate a very narrow read width.